Noise reduction circuit



Patented Oct. 10, 1944 NOISE REDUCTION cracm'r Robert L. Haynes, Indianapolis, Ind., assignor to Radio Corporation of America, a corporation of Delaware Application December 31, 1942, Serial No. 470,791

.(Cl. 179l00.3)

9 Claims.

This invention relates to sound recording systems and particularly to the noise reduction circuits of such recording systems.

It is well known that in the recording of sound either by the variable density or variable area methods, a'portion of the recording system in cludes noise reduction circuits which generally comprise an A. C. amplifier, a rectifier, timing and filter sections, and a direct current amplifier. Since the noise reduction currents are employed to either operate a shutter or shutters to eliminate light from a portion of the sound track area not being utilized for sound modulations, or vary the average intensity of a recording lamp in accordance with the level of the signals being recorded, it is essential that the shutters be moved or the average intensity be varied in a manner which prevents the introduction of distortion into the reproduced sound. Specifically, this distortion ma be produced by the noise reduction elements being varied too rapidly at the beginning of a signal wave causing an audible thumpin the reproduced sound. Distortion is also introduced by the elements being varied too slowly causing the first modulations of the signal to be clipped,

Thus, since too rapid ,an action introduces thumps and too slow an action introduces peak clipping, compromise is desired. Also, the particular movement of the shutters or variation in the average light intensity should have a definite for the noise reduction shutters or for the variation in the average light intensity.

A further object of the invention is to maintain the optimum action for noise reduction elements while obtaining optimum filtering of the noise reduction currents. I

A further object of the invention is to provide a double rectifier unit for a noise reduction system wherein the fast opening and closing characteristics are obtained together with improved filtering.

Although the novel features which are believed to be characteristic of this invention are pointed out with particularity in the claims appended herewith, the manner of its organization and the mode of its operation will be better understood by referring to the following description, read in conjunction with the accompanying combination schematic and diagrammatic drawing of'a sound recording system embodying the invention.

The usual type of sound recording system is shown having a microphone 5 feeding an amplifier 6, the output of which is impressed upon a sound recorder l of either the variable area or variable density type. Bridged across the output of the amplifier \6 is a pair of conductors I0 feeding a noise reduction amplifier I I, the output of which is connected to the primary l2 of a transformer l 3.

The transformer [3 has a tapped secondary l5;

\ the-tapped points I6 of which are connected to predetermined rate of movement after a signal ceases or when the signaldrops from a high amplitude level to a lower one (closing time);

However, since the noise reduction elements are.

actuated by rectified signal currents, the filtering must be suirlcient to prevent the products of rectification from being impressed on the noise reduction elements to introduce further distortion.

In general, filtering may be improved by reducing the rate of discharge or faster discharge may be obtained at the expense of filtering. If the usual filtering is maintained, the present invention will decrease the discharge time while if normal timing is maintained, the invention will improve the filtering. Thus, a second compromise made be made correlated with the opening time. or charging characteristics.

The principal object of the invention, therefore, is to improve the noise reduction portion of a sound recording system.

Another object of the invention is to provide a noise reduction rectifier and filter system which produces the proper opening and closing-rates reduction elements in the recorder I, a shutter system of this type being shown'i'n U. S. Patent Nos. 2,102,776, 2,102,777 and 2,102,778.,

As mentioned above, the particular circuit shown in detail including the transformer l3, rectifiers l8 and I9, condenser 20, and resistances 2i and 22, provides a particularly desirable opening-and closing rate for the noise reduction elements, together with the necessary filtering of the rectified currents. This result is obtained by the use of the rectifier I9 upon which is impressed a higher voltage than that impressed upon the normal rectifier It. If the rectifier I9 were reto the standard oise reduction circuit operating full wave, the va ue of the resistances 2| and 22 being a compromise between the conflicting re-- quirements of optimum opening and closing times and good filtering.

In former circuits, the condenser 20 was charged by the rectified voltage obtainedbetween the center tap of the transformer i3 and taps l6. At the cessation of the signal the discharge of the condenser 20 through a single shunt re-' sistance controlled the noise reduction elements. The present invention splits this discharge control resistance into two parts 2| and 22, resistance 2| being from ten to one hundred times larger than resistance 22, the combined value of resistances 2| and 22 being equal to the single shunt resistor in the normal circuit Typical values for these elements are 225,000 ohms for resistor 2| and 25,000 ohms for resistor 22, while the timing condenser 20 is .03 mf. Without rectifier I9, these elements will provide a discharge time constant of .0075 second. To provide sumcient filtering, however, the present circuit instead of returning the discharge path to ground,

returns it to the plates .of rectifier M, which follow the instantaneous rectified potential across the full secondary winding of the transformer Hi, this rectified voltage always being negative except at the instants when it is zero.- Thus, the discharge of condenser 20 between peaks is much less than it would be if the discharge resistor were grounded,

The above described circuit may be considered as a single resistance-capacity filter fed by two rectifiers at two different points in such a way that for constant and slowly changing signal levels it acts as an averaging. type of' rectifier having a resistance input to the filter, slow timing, and consequently, good filtering- During rapid signal increases, however, the circuit acts as a condenser input, peak reading. circuit which provides fast opening.

Specifically, let us assume rectifier I8 is out of the circuit and a steady sine wave input is such as to produce a peak voltage of 10 volts between one of terminals I6 and the center tap on the secondary of transformer |3.-- With the proper transformer winding, a peak voltage of 15.7 volts can be produced between one of the outside terminals andthe midtap (ratio 11-72). Now with rectifier l8 removed, the circuit operates as a full wave rectifier with a resistance capacity filter. having a-high resistance input. This circuit produces a well-filtered direct current output having the average value of the applied sine wave. This is 10 volts for a'wave of 15.7 volts peak. Thus rectifier it alone can maintain thelO volt output and if rectifier; 8 is connected, into the circuit, it will carry no current because only 10 volts peak signal is applied to it between one of terminals i8 and themidtap. This condition will hold true for constant signals, decreasing signals and slowly increasing signals. In other words, for constant or slowly changing signal levels, the output of the circuit is produced entirely by an averaging .circuit which has very good filtering properties.

For rapidly increasing signals rectifier I8 is effective since" at these times the voltage applied to rectifier I8 is greater than the voltage-across condenser 20. This is caused by the charging rate of condenser 20 by rectifier I! being delayed by resistance 2|. The result is a fast opening action during rapid increases in the signal levels.

2,859,989 moved, the remaining circuit would be equivalent Thus, the desired opening and closing characteristics may be obtained combined with good filtering.

Considerable improvement has been obtained even when both rectifiers are fed from the same transformer taps, but better filtering is produced when rectifier I9 is fed with a higher voltage in the manner shown in the drawing. For best filtering the voltage impressed on the rectifier l9 should be about 5'? percent higher than the voltage impressed upon rectifier l8. The rectifier l8 may also be a half-wave, rectifier employing only one of the taps l6, although for best operation rectifier l9 should be full-wave.

The power requirements of this circuit are not increased appreciablyby the addition of the rectifier l9, this rectifier being loaded only by the resistance 22 which may have a value of 5,000 to 25,000 ohms which requires very little power input.

I claim as my invention: i

1. In a sound recording system, a sound recorder including a noise reduction unit comprising an amplifier for amplifying the noise reduction signal, a transformer, a rectifier connected to said transformer 'for rectifying said noise reduction signal, means for impressing said rectified signal upon the noise reduction elements of said recorder, a timing condenser and a resistance in shunt thereto for controlling the rate of increase and" decrease of the said rectified currents with increases and decreases in said signal, and means interconnected between a portion of said resistance and said transformer fonvarying the discharge rate of said timing condenser.

2. A sound recording system in accordance with claim 1 in which said last-mentioned means includes a second rectifier. I

3. A noise reduction unit comprising a signal amplifier, a rectifier connected thereto, a direct current amplifier for the products of rectification, a timing circuit including a resistance and condenser intermediate said rectifier and said direct current amplifier, a transformer intermediate said signal amplifier and said rectifier, and means connected intermediate said transformer and a portion of said resistance for varying the discharge rate of said condenser.

4. A noise reduction unit in accordance with claim 3 in which said last-mentioned means includes a second rectifier, said second rectifier being loaded by said portion of said timin i resistance. g c mum 5. A noise reduction unit in accordancejwith rectifier being greater than the voltage impressed on said first rectifier.

6. A noise reduction unit comprising a transformer, a plurality of rectifiers connected to said transformer, a timing circuit including a shunt condenser and resistance elements, and a filter circuit connected to said timing. circuit. one of said rectifiers being connected directly to .said

timing condenser and to a portion of the secondary of saidtiansformer, and another of said rectifiers being connected to said timing condenserthrough one of said timing resistance elements and to the'full secondary of said transformer.

7. A noise reduction unitin accordance with claim 6 in which saids'econd mentioned rectifier is loaded by another 'of "said "timing resistance of said timing circuit resistance, the voltage impressed on said second interconnecting said A. C. amplifier and said elements. said resistance elements being connected in series.

8. A noise reduction unit comprising a variable tapped transformer, a timing circuit including a shunt condenser and shunt resistance elements connected in series, a rectifier connected.

between a portion of said secondary and directly across said condenser and resistance elements, and a second rectifier connected across the full secondary or said transformer and one of said resistance elements.

9. A noise reduction unit comprising a plurality of rectifiersJ Lfan A. C. amplifier, a transformer 

